1. Field of the Invention
The present invention relates to an improved wire feed apparatus and more particularly to a novel electrical circuit for sensing the presence of a second bond and for monitoring the continuity of bonding wire between the bonding tool and the wire clamps of an automatic wire bonder.
2. Description of the Prior Art
Automatic wire bonders are classified in Class 228, subclasses is 1 to 4.5. Wire feed mechanisms for fine bonding wires employed on such wire bonders are classified in International Class H 01 L, subclasses 21/60 and 23/48. The present invention is adapted to be incorporated into an existing Model 1471 automatic wedge wire bonder which is commercially available from Kulicke and Soffa Industries, Inc. in Willow Grove, Pa. 19090.
Automatic wire bonders are adapted to interconnect fine wire between the electrodes or terminals of semiconductor chips with the lead out pads on carriers or substrates without the necessity of an operator observing or controlling the bonding machine. Needless to say, numerous conditions can occur which can cause a malfunctions or improper bonds. If such malfunctions are permitted to proceed without stopping the automatic wire bonder, a very large number of semiconductor chips may be harmed beyond repair. It has been recognized heretofore that protective means must be incorporated onto automatic wire bonders to prevent destruction of valuable semiconductor chips.
Heretofore, circuits have been designed for detecting missing wires in wire feed systems of automatic wire bonders. A typical example of such a circuit is shown in U.S. Pat. No. 4,485,957. This device detects the continuity between the wire on the wire spool and the bonding wire at the wire clamps. If the bonding wire is in the wire clamps during the clamping operation, the circuits sense the space caused by the presence of the wire, however, such circuits do not detect the presence or the absence of the bonding wire under the bonding tool.
U.S. Pat. Nos. 4,558,596 and 4,586,642, assigned to Kulicke and Soffa Industries, Inc., teach circuits for monitoring the presence of the bond and the condition of the bond. Such circuits employ statistical data to enhance the result of the bond monitors. However, such circuits require an accumulation of statistical data as well as experience with the type and size of fine bonding wire being employed.
The prior art detecting circuits detect missing bonds as well as bad bonds and will stop the automatic wire bonder. Some of the bonds that are deemed to be acceptable are detected as being imperfect or improper bonds and the automatic wire bonder is stopped until a maintenance operator can examine the reason for the stoppage and restart the automatic wire bonder. The present invention is not concerned with detecting the quality of the bond being made but is concerned with detecting if a bond has been made and whether wire is being consumed during the bonding operation which is indicative of proper operation of the automatic wire bonder. Thus, the improved sensing or monitoring circuit is capable of producing greater output without producing any scrap. The present sensing and monitoring circuit does not stop the wire bonder unless wire bonds are not being made and/or no fine wire is being consumed while making the wire bond interconnections.
It is desirable to provide an improved monitor and sensing circuit for detecting the continuous presence of bonding wire at the bonding tool of an automatic wire bonder. In the absence of the simple foregoing conditions, the automatic wire bonder will stop all wire bonding operations without proceeding to destroy the semiconductor device being wire bonded.